The present invention relates to a rotary fluid machine such as, for example, a screw compressor, a centrifugal compressor, or a pump, and, more particularly, to a rotary fluid machine which includes a rolling thrust bearing mounting means and which is suitable for use as a oil-free screw compressor in which a pair of screw rotors rotate in a meshed state.
During operation of a rotary fluid machine, a radial load and a thrust load act on the rotors thereof and, in order to support these loads, radial bearings and thrust bearings are provided on the trunnions of the rotors.
In, for example, U.S. Pat. Nos. 3,388,854 and 3,932,073, screw compressors are proposed wherein radial bearings and thrust bearings are provided for supporting radial and thrust loads, with the thrust bearings supporting a radial load in addition to the thrust load.
Preferably, in screw compressors, the thrust bearings support only the thrust load since, when the thrust bearings also support the radial load, the natural frequencies of the bending vibration of the rotors vary as compared to a screw compressor in which the thrust bearings do not support a radial load. Moreover, it is difficult to determine numeric values of the natural frequencies of the bending vibration of the rotors after they have thus changed. Consequently, the rotary fluid machine operates with unknown natural bending vibration frequencies of the rotor and it is dangerous to operate a rotary fluid machine in this condition since there is a very high possibility that resonance will occur. Since the resonance generates abnormal vibrations, the lifetime of the bearings is considerably reduced and eventually the vibrations destroy the thrust bearing mounting unit.
The aim underlying the present invention essentially resides in providing a rotary fluid machine such as, for example, a screw compressor, rotary compressor, pump or the like with a thrust bearing mounting unit in which thrust bearings for supporting a thrust load of the rotary body bear substantially no radial load thereof.
In accordance with advantageous features of the present invention, a rotary fluid machine is provided which includes a casing having a rotor-housing space and an inlet and outlet passage communicating with the space. At least one rotor is housed in the space and is rotatably supported on the casing by radial bearing means and thrust bearing means. A gas entering the space through the inlet passage passes the rotor and is then discharged through the outlet passage. Each thrust bearing means have inner races mounted securely onto a shaft portion of the rotor and outer races fixed to the casing by a bearing support unit, with the support unit having an axial rigidity at least as high as the thrust bearing means and a radial rigidity not more than half thereof so that the thrust bearing means bears substantially no radial load.
In accordance with further advantageous features of the present invention, the support unit has a bearing member provided with a flange and a plurality of studs with which the bearing member is fixed to the casing. The studs are provided between the casing and the flange of the bearing member and are separated by equal angular distances from one another. Each of the studs have an axially extending smaller-diameter portion which is formed such that the axial length thereof is greater than the thickness thereof.
In accordance with further advantageous features of the present invention, the support unit includes a bearing member provided with a flange and a fastener means by which the bearing member is fixed to the casing, with the bearing member having a plurality of columnar portions which extend axially from the flange so that the end portions thereof are in contact with end surfaces of outer races in the thrust bearing means. Each of the columnar portions are formed so that an axial length thereof is greater than a thickness thereof.
It is also possible in accordance with the present invention for the support unit to include a bearing member provided with a flange and a plurality of first columnar portions extending axially from the flange, with fastner means being provided for fixing the bearing member to the casing. A plurality of second columnar portions may extend axially from an oil feed ring, with the outer races in the thrust bearing means being positioned between the first columnar portions and the second columnar portions and being supported and held between the first and second columnar portions.
The support unit may, in accordance with the present invention, be provided with a housing holding the outer races of the thrust bearing means with the flange being attached to the housing. Advantageously, the housing may be attached to the columnar portions.
The studs may, for example, include a first screw portion, a smaller diameter portion attached to the first screw portion as well as a second screw portion, with the first screw portion being driven into the case. A nut means or the like may be provided for fixing the flange to the stud being threadably secured onto the second screw portion.
In accordance with further features of the present invention, the total number of columnar portions may be three in number.
In accordance with additional features of the present invention, a pair of rotors may be housed in the working space, with each of the rotors having a toothed portion and shaft portions. Roller radial bearing means may be provided between the shaft portions of the rotors and the casing to rotatably support the rotors. Roller thrust bearing means rotatably support the shaft portions of the rotor, and a bearing support unit may be provided between the roller thrust bearing means and the casing, with the bearing support unit having an axial rigidity at least as high as that of the roller thrust bearing means and a radial rigidity not more than half thereof.
The shaft portions of the rotors may be provided with timing gears which rotate the rotors in synchronism and a shaft sealing means may be provided between the roller bearing means on the shaft portions and the toothed portions of the rotors.
By virtue of the advantageous features of the present invention, the outer races of the rolling thrust bearings are fixed to a casing by a bearing mounting unit, with the bearing mounting unit being constructed so that an axial rigidity thereof is higher than that of the rolling thrust bearings, and the radial rigidity thereof is considerably lower by, for example, less than half, of the latter.
The radial rigidity of thrust bearings fixed to the casing by the bearing mounting unit can be determined by the following equation: EQU 1/K=1/K.sub.B +1/K.sub.H ;
where:
K.sub.H =1/2K.sub.B, PA1 K is 1/3K.sub.B, PA1 K.sub.H =1/4K.sub.B, and PA1 K=1/5K.sub.B.
In the above equation, K.sub.B and K.sub.H, respectively, represent the radial rigidity of the thrust bearings and the radial rigidity of the bearing mounting unit, with K representing the total rigidity. Consequently, in order to achieve the objects of the present invention, it is necessary for the radial rigidity of the bearing mounting unit to be no more than half that of the thrust bearings.
Accordingly, it is an object of the present invention to provide a rotary fluid machine with a thrust bearing mounting unit which is capable of maintaining the natural frequencies of bending vibration of the rotors within a predetermined range so as to prevent any dangerous operation of the machine.
Still another object of the present invention resides in providing a rotary fluid machine with a thrust bearing mounting unit capable of preventing the occurrence of abnormal vibrations, even when the rotors are vibrating and/or causing other components of the fluid machine to resonate.
A further object of the present invention resides in providing a rotary fluid machine with a thrust bearing mounting unit which extends the normal lifetime of the bearings.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, several embodiments in accordance with the present invention.